Submicron particle removal from gas streams

ABSTRACT

Disclosed are methods and systems for removing submicron particles from a gas stream, in particular from urea prilling off-gas, wherein a Venturi ejector is used. A method comprises contacting a gas stream containing submicron particles in a Venturi ejector with an injected high velocity scrubbing liquid to provide a pumping action, wherein the scrubbing liquid has an initial velocity of at least 25 m/s and wherein the ratio of scrubbing liquid and gas flow is between 0.0005 and 0.0015 (m3/h)/(m3/h).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/495,667 filed 24 Apr. 2017, which claims priority under 35 U.S.C. 119to European patent application No. 16168796.7 filed 9 May 2016.

FIELD OF THE INVENTION

The invention pertains to removal of particles from a gas stream, inparticular to removing submicron urea dust from a gas stream from a ureaprilling tower.

BACKGROUND

Removal of submicron particles from gas streams is often critical forensuring compliance with emission limits for many industrial processes,such as urea finishing. For instance off-gas from urea prilling towerscontains a relatively large amount and/or large fraction of submicronparticles, for instance compared to the off-gas from a urea fluid bedgranulator. Hence, the removal of submicron urea particles isparticularly important in order to meet ever more stringent regulationsand limits on the emission of urea. The removal of ammonia from ureafinishing off-gas is also necessary. Background references relating tothe removal of urea dust from off-gas from a urea finishing sectioninclude WO 2015/002535 and WO 2015/072854.

It may be observed that particle size distribution of urea prillingtower off-gas has a peak between 0.1 μm and 1 μm aerodynamic particlesize, with a cumulative mass of for example about 70 mg/Nm³ provided byparticles<10 μm (about 50 wt. % total particulate matter, PM). Off-gasfrom urea granulation may for example contain about 25 mg/Nm³ ofparticles<10 μm. For compliance with current and future emission limits,significant removal of submicron particles, e.g. urea dust, isessential. Available particle capture technologies generally have verylow efficiency for submicron particles or have a large pressure drop.

In urea prilling, urea melt is supplied at the top of a prilling tower,and distributed as droplets. The urea melt droplets solidify as theyfall down while cooling against a large quantity of upward-moving air.Urea prills are withdrawn from the bottom. The fresh cooling air entersthe bottom of the prilling tower. The off-gas comprising urea andammonia leaves the prilling tower near the top.

A prilling tower can for instance have a height of for example 60 m to80 m. Smaller plants may have a free fall path of 50 m or less. Some ofthe largest plants have prilling towers of 125 m height. Emissions canfor example be 0.5 to 2.5 kg urea dust per ton urea prills (35 to 125mg/Nm³) and about 0.5 to 2.7 kg NH₃ per ton (35-245 mg/Nm³). Urea dustemissions of more than 200 mg/Nm³ have been reported for some existingurea prilling towers. An example indicative air flow for a urea prillingtower is 500 000 Nm³/hr. A larger urea prilling tower may for instancehave 900 000 Nm³/hr, with a urea capacity of 75-100 mt/hr (metric tonper hour).

Older prilling towers frequently vent off-gas directly to air withoutany urea or ammonia abatement. The tower construction generally sets amaximum for the weight for the design of any abatement systems installedon top as part of revamping. The off-gas from some prilling towers has alow pressure drop, in particular the off-gas from prilling towersoperating with natural draft. Existing emission abatement technologiestypically would require large blowers and fans to maintain sufficientpressure drop, since generally submicron particle removal requires highpressure drop. Current systems are hence not suitable for installationon top of an existing prilling tower. The possibility of first bringingthe off-gas to lower elevation through a duct would introduce anadditional significant pressure drop. In view of the large airflows thiswould lead to a significant increase of the power consumption. Theconstruction of a duct from the top to the bottom of the urea prillingtower is also challenging and expensive, and introduces the risk ofplugging in the duct between the prilling tower and the emissionabatement system.

Dust scrubbers, sometimes combined with an acid washer to reduce ammoniaemissions, are generally considered with currently available emissionabatement systems to be feasible only for forced draft prilling towerswere air fans are available.

Accordingly, there is a need for more effective emission abatementsystems and methods that can operate with a low pressure drop and caneffectively remove submicron particles from gas streams. More inparticular, there is a need for better urea and ammonia emissionabatement systems and methods for urea prilling towers.

SUMMARY OF THE INVENTION

In order to better address one or more of the above mentioned desires,the invention pertains, in an aspect, to a method for removing submicronparticles from a gas stream, the method comprising: contacting a gasstream containing submicron particles in a Venturi ejector with aninjected high velocity scrubbing liquid to provide a pumping action,wherein the scrubbing liquid has an initial velocity of at least 25 m/sand wherein the ratio of scrubbing liquid and gas flow is between 0.0005and 0.0015 (m³/h)/(m³/h).

The invention also pertains, in an aspect, to a gas treatment systemcomprising a Venturi ejector.

A further aspect pertains to a method of modifying existing plants, inparticular urea prilling towers, comprising addition of a Venturiejector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a block diagram of an embodiment of a system of theinvention.

FIG. 2 depicts a block diagram of an embodiment of a system of theinvention.

FIG. 3 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a prilling tower.

FIG. 4 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a urea granulator.

DETAILED DESCRIPTION

The method for removing submicron particles from a gas stream comprisescontacting the gas stream containing submicron particles in a Venturiejector with a scrubbing liquid. The scrubbing liquid is preferablyinjected, and preferably injected with high velocity, in particular toprovide a pumping action. In this way the gas stream to be treated isdrawn into the Venturi ejector. The scrubbing liquid preferably has aninitial velocity of at least 25 m/s, more preferably at least 50 m/s,even more preferably 100 m/s. Such velocities refer for example to thevelocities at a nozzle opening and/or to the mean droplet velocity overe.g. 1 cm from the nozzle. Preferably, the method comprises injectingscrubbing liquid in a Venturi ejector with a hydraulic nozzle, such as ahigh pressure hydraulic nozzle, e.g. with a nozzle pressure of at least15 bar or at least 18 bar, or with a dual fluid (gas and liquid) nozzle,which may have a pressure lower than 15 bar, in order to provide suchpreferred velocities.

Preferably, scrubbing liquid is injected in a Venturi ejector so as togive a mean droplet diameter of less than 300 μm, more preferably lessthan 200 μm. In some embodiments, the method comprises injectingscrubbing liquid in a Venturi ejector through a nozzle configured toprovide droplets with a mean droplet diameter of less than 300 μm,preferably less than 200 μm. Droplet sizes are e.g. Volume MedianDiameter. This droplet size may contribute to efficient scrubbing.Scrubbing liquid droplets of this size can for example be provided byusing the mentioned scrubbing liquid velocities. In particular ahydraulic nozzle with an injection pressure of at least 15 bar can beused, more preferably at least 18 bar or a dual fluid injector whichuses compressed gases, of e.g. 3 to 6 bar, to finely atomize thescrubbing liquid, of e.g. 3 to 6 bar.

Preferably, the ratio of scrubbing liquid and gas flow is between 0.0005and 0.0015 (m³/h)/(m³/h) in at least one Venturi ejector or in eachVenturi ejector stage. Also possible is a ratio of scrubbing liquid togas flow such as 0.00010 to 0.0050 (m³/h)/(m³/h), or lower or higher.The ratio is for example based on actual m³ of the gas stream to bedrawn into the Venturi ejector. Optionally, the scrubbing liquid/gasratio is in the range of 0.5 to 1.5 l/m³, based on actual m³.

Preferably, the method comprises using a plurality of Venturi ejectorsin series. Preferably, the residence time of the gas stream between thefirst and the downstream second Venturi ejector is at least 0.1 s or atleast 0.20 s or at least 0.4 s, for example more than 0.8 s. Thisrelates to the residence time between two Venturi ejectors without otherVenturi ejectors between them. Other elements may be present betweenthem, such as a sprayer or a mist eliminator.

Preferably, a first upstream and a second downstream Venturi stage areused each comprising one Venturi ejector or a plurality of parallelVenturi ejectors. Preferably, the first scrubbing liquid of the firstVenturi stage comprises at least 10 wt. % or at least 20 wt. % or atleast 30 wt. % or at least 40 wt. % dissolved material, such as 20 to 55wt. % or 40 to 50 wt. %, and/or such amounts of hydrophilic material, orsuch amounts of material of components removed from the gas stream.Preferably, the dissolved material is urea and the scrubbing liquidcomprises such amount of urea. In some embodiments, the first scrubbingliquid comprises less than 90 wt. % water, or less than 80 wt. %, oreven less than 60 wt. %. The scrubbing liquid used in the seconddownstream Venturi stage and/or in a spray nozzle downstream of thefirst Venturi stage preferably comprises 0 wt. % to 5.0 wt. % dissolvedmaterial, more preferably less than 2.0 wt. % dissolved material, inparticular for urea. Preferably, the second scrubbing liquid comprises80 wt. % to 100 wt. % water, or at least 90 wt. %, or at least 95 wt. %water. Preferably, the scrubbing liquid used in the first Venturiejector stage (first scrubbing liquid) has a higher concentration ofdissolved material than the scrubbing liquid used in the second Venturiejector stage (second scrubbing liquid), preferably at least 3 timeshigher or at least 5 times higher or more preferably at least 10 timeshigher. The scrubbing liquid used in the first Venturi stage, inparticular in the most upstream Venturi ejector, is for example thefirst point of contact with the off-gas.

The first scrubbing liquid is generally recirculated to provide suchconcentrations of dissolved material (e.g. urea) so as to allow foreasier disposal, in particular urea recovery. In particular in case ofoff-gas (cooling air flow) from prilling or granulation, the prilled orgranulated material is preferably recovered. This applies in particularto urea granulation or prilling. The recovered urea may be incorporatedin a urea-containing product, e.g. the prills or granules. Generally, astream comprising such concentrations of urea, e.g. 40 wt. % to 50 wt. %is withdrawn from the first Venturi stage and/or an upstream quenchstage as purge and/or blowdown stream, in particular from the collectionbasin or recirculation loop of these. Hence, the bulk of the particulatematerial in the gas stream is for example captured by scrubbing withscrubbing liquid comprising such high concentrations of dissolvedmaterial.

Introducing a liquid having a lower dissolved material concentration(e.g. urea concentration) than used in a first Venturi stage in the gasstream downstream of the first Venturi ejector may result in an increaseof the partial water vapor pressure in the gas stream. This may promotecondensation of water on submicron particles, resulting in an increaseof the particle size. This can improve capture of the now largerparticles in a downstream particle and/or droplet capture device, suchas for example (a part of) a Venturi ejector (e.g. a diverging tubepart) and/or mist eliminator. In particular condensation on submicronparticles and/or aerosol droplets containing urea in relatively higherconcentration, such as at least 50 wt. %, or even 100% urea, can bepromoted. In some embodiments, the method comprises evaporating at least0.001 kg/Nm³ or at least 0.005 kg/Nm³ or at least 0.010 kg/Nm³ waterdownstream of a first Venturi ejector stage and upstream of the throatof a Venturi ejector and/or droplet removal device.

Preferably, the gas stream is obtained from a urea finishing section,such as a urea granulator or a urea prilling tower, more preferably aurea prilling tower. The urea prilling tower is for example a forceddraft or a natural draft urea prilling tower. The present method andsystem are especially advantageous for use with natural draft ureaprilling towers. Optionally, the process further comprises a step ofurea prilling or urea granulation. Optionally, the process comprisessolidification of a urea melt to produce urea prills or granules, usingair for cooling of the urea melt droplets.

In some embodiments, the gas stream contains a concentration ofsubmicron particles greater than 20 mg/Nm³, or greater than 50 mg/Nm³,more preferably such concentration of urea particles. Submicronparticles have a particle size of 1.0 μm or less. Optionally, thepercentage of submicron particles is at least 0.5 wt. % and/or at most5.0 wt. % of the total weight of the particles in the gas stream,preferably of particles smaller than 1.0 μm, and optionally the amountof such particles is in the range 1.0 to 4.0 wt. %.

Preferably, the submicron particles are hydrophilic. Preferably, thesubmicron particles comprise a hygroscopic material. Preferably, thesubmicron particles are dissolvable in the scrubbing liquid, e.g. inwater. As used herein, submicron particles encompass for examplecolloidal aerosols. Condensation may optionally involve condensationonto a particle, droplet, or colloidal aerosol, causing it to grow insize.

The Venturi ejector as used herein is a type of Venturi scrubber andgenerally comprises, consecutively in series in the direction of the gasflow, a converging tube part, a throat, and a diverging tube part,wherein the converging and diverging part are usually conical tubeparts. The throat usually provides a narrow opening for passage of thegas stream and liquid supplied into the gas stream upstream of thethroat. The throat may be provided for example by a joint between twoparts, e.g. tube parts, or for example by a tube internal cross-sectionminimum. The acceleration and/or high velocity in the throat and/orconverging part contributes to intimate mixing of the gas and liquid,and to turbulence and atomization of the liquid. At least some particlesin the gas stream impact on droplets and are entrained and may beremoved in a downstream droplet removal device.

Preferably, the Venturi ejector (e.g. ejector Venturi scrubber)comprises a nozzle positioned for spraying scrubbing liquid in adirection parallel with the gas flow (of the gas stream to be treated)through the gas inlet of the Venturi ejector. Optionally, the centerlineof the nozzle is parallel to the gas flow. Preferably, the nozzle isinserted into a converging part, such as a conical tube part, of aVenturi ejector. In some embodiments, the nozzle is spaced apart from awall of a Venturi tube or duct part. Preferably, the gas inlet of theVenturi ejector is an opening extending substantially perpendicular(e.g. at an angle between 60° and 120° or between 85° and 95°) to theline from the nozzle to the throat of the Venturi. Preferably, the gasinlet of the Venturi ejector is arranged substantially parallel to theopening of the throat. Preferably, the gas flow centerline of the gas tobe treated does not bend between nozzle and throat (irrespective ofconverging of the flow). Preferably, the nozzle is positioned forspraying perpendicular to the throat cross-section and preferably thenozzle is centered to the throat cross section. Generally, the nozzle isspaced and upstream from the throat cross section. In some embodiments,liquid is supplied into a round throat Venturi ejector only through onesuch nozzle.

The nozzle used to introduce the scrubbing liquid in a Venturi ejectoris for instance hydraulic, producing small droplets through highpressure, or for example a dual fluid nozzle, wherein a liquid and anauxiliary gas stream, typically at pressure, flow both through thenozzle. Small droplets can be produced through shear forces between theliquid and gas that both travel through the nozzle.

The scrubbing liquid spray acts as motive fluid for the Venturi ejector,together with the air stream in case of a dual fluid nozzle. Hence, theVenturi ejector may acts as Venturi eductor wherein the gas stream to betreated is drawn in with the motive fluid stream. It may be noted thathigh energy Venturi scrubbers (with initial liquid velocity lower thanthe gas velocity) and ejector Venturi scrubbers (initial liquid velocityhigher than the gas velocity) have entirely different energyconsumption, atomization and scrubbing characteristics. The presentinvention involves ejector type Venturi scrubbers. The kinetic energy ofthe high velocity liquid (with or without co-injected gas flow) is usedto atomize the liquid and to pump the gas stream to be treated,generally through the scrubbing system and connecting ducts. The Venturiejector is generally used with a downstream droplet eliminator, e.g. agravity or inertial impact separator to remove scrubbing liquid from thegas stream. In particular a downstream mist eliminator can be used.

Optionally, a basic reagent is added, for example selected from thegroup consisting of: caustic, lime, limestone, hydrated lime, fly ash,magnesium oxide, soda ash, sodium bicarbonate, sodium carbonate, andmixtures thereof. This may be used for removal of acidic gases from thegas stream. Preferably, the reagent is added to a scrubbing liquidsprayed into the gas stream. Preferably the scrubbing liquid of aVenturi ejector comprises such reagent.

Preferably, an acidic reagent is added, more preferably selected fromthe group consisting of: acetic acid, boric acid, carbonic acid, citricacid, hydrochloric acid, hydrofluoric acid, nitric acid, oxalic acid,phosphoric acid, sulfuric acid, and mixtures thereof. This may be usedfor removal of basic gases from the gas stream, such as ammonia.Preferably, an acidic reagent is added in case of urea finishingoff-gas. Preferably, sulfuric acid or nitric acid is added. Preferably,the scrubbing liquid of a Venturi stage comprises such acidic reagent,in particular of the first (most upstream) Venturi stage.

Preferably, the acid or basic reagent is added to a scrubbing liquidsprayed into the gas stream, preferably downstream of a first Venturistage and optionally also downstream of a second Venturi stage.Preferably acid or basic reagent is comprised in a scrubbing liquid of aVenturi ejector, for instance of a first or second or optional thirdVenturi stage. A scrubbing solution from an acid scrubbing stepcomprising ammonium salt is for example supplied to a holding tankand/or outside battery limits, in particular if the acid reagent isintroduced into the gas stream downstream of the first Venturi stage.

Preferably, the method involves acid scrubbing and dust scrubbing ofurea prilling off gas, preferably carried out on top of a preferablynatural draft urea prilling tower, i.e. in an abatement system locatedat the top of a urea prilling tower.

In some embodiments, a stream comprising dissolved urea, such as ablowdown and/or purge stream, for example from the first Venturi stageand/or a quench stage wherein a scrubbing liquid with or without acidicor basic reagent is used, is supplied to a recycle vacuum evaporationsection, to give water vapor and a concentrated urea solution. Therecycle vacuum evaporation section is preferably separate from andadditional to the evaporation section of a urea plant. The concentratedsolution comprising urea is supplied to (a stream to) the urea finishing(e.g. granulation or prilling) and the urea is included in the solidurea product, e.g. granules or prills. The vapor is condensed and thecondensate is preferably reintroduced in the described method as make upwater, e.g. used for scrubbing the prilling off-gas with an aqueoussolution comprising less than 5 wt. % urea, such as in the secondVenturi stage. In case the process comprises acid scrubbing, the streamand concentrate may further comprise ammonium salts. A concentrate mayalso be supplied to a urea ammonium nitrate (UAN) plant or urea ammoniumsulphate (UAS) plant and introduced into a UAN or UAS product stream. Insome embodiments, the method comprises acidic scrubbing downstream of afirst Venturi stage and/or first quench or scrubbing stage and theacidic scrubbing solution purge stream is disposed of separately fromthe scrubbing liquid used upstream of said acidic scrubbing. Thescrubbing liquid used in a stage upstream of the acidic scrubbingcomprising urea is subjected to such evaporation.

Preferably, the static (absolute) pressure at the exit of the Venturiejector is nearly the same or slightly larger relative to the gasentrance of the Venturi ejector, e.g. at least 90% or at least 100% orat least 105% of the static pressure at the entrance. Preferably, themethod comprises urea prilling in a natural draft urea tower and Venturiscrubbing with said Venturi ejector on top of said prilling tower,wherein the static pressure at the exit of at least one Venturi ejectoris larger than at the entrance of the Venturi ejector.

Preferably, at least some or all Venturi ejectors are arrangedsubstantially horizontally, e.g. for substantially horizontal flowthrough the throat, for instance at an angle of less than 20° or lessthan 10° to horizontal. This allows for a compact design. Also possibleis for instance that at least some or all of the Venturi ejectors have avertical orientation. In that case, the Venturi ejectors are arrangedfor flow downward through the throat. This allows for a small pressuredrop.

Optionally, the method comprises quenching the gas stream upstream ofthe first Venturi ejector, for example by a temperature decrease of atleast 10° C. or at least 20° C. or to a gas temperature of less than 60°C., or of 50° C. or less, for instance by spraying an aqueous solutionand evaporation of at least some water. Optionally, the quenching spraysolution comprises at least 10 wt. %, or at least 20 wt. % or at least30 wt. % dissolved material, e.g. urea. Optionally, the quench spraysolution is at least in part obtained from recirculated Venturiscrubbing liquid, such as of a first Venturi stage. Optionally, thequench spray solution essentially consists of water. The quench solutionis for example sprayed as fine mist and/or in cross flow or co-currentflow.

Optionally, the gas is passed through a droplet removal device, such asa demister, between a first and second Venturi stage and/or after asecond Venturi stage. Optionally the method comprises further downstreamparticle capture and/or gas treatment steps.

In some embodiments, the method is carried out at the top of a prillingtower, more preferably on top of a urea prilling tower. Preferably, atleast the Venturi ejector is placed on or at the top of a prillingtower, in particular a urea prilling tower.

Optionally, the method comprises one or more scrubbing steps, comprisingscrubbing the gas stream with a scrubbing liquid, e.g. by spraying. Themethod optionally comprises passing the gas stream through a thirdVenturi stage, e.g. a third Venturi ejector or a (high energy) Venturiscrubber. A third Venturi stage may for instance be positioned betweenthe first and second Venturi ejector stage or downstream of the secondVenturi ejector stage. An optional third Venturi stage may for instanceoperate with a scrubbing liquid comprising an acid reagent. In someembodiments, the draw is essentially provided by the Venturi ejectors.In the method, submicron particles are removed from the gas stream.Generally, particles larger than 1 μm are also removed. Soluble gasessuch as ammonia may also be removed.

In a preferred embodiment, the method comprises subsequently:

A. providing a gas stream, preferably comprising or essentiallyconsisting off-gas from a urea finishing section, more preferably a ureaprilling tower, preferably from a natural draft or forced draft ureaprilling tower,

B. spraying an aqueous solution into the gas stream, preferablycomprising 20 wt. % to 55 wt. % urea, optionally to cool by at least 10°C. or at least 20° C. or to a temperature of less than 50° C.,preferably in cross-flow or co-current flow,

C. passing the gas stream through a first ejector Venturi scrubberhaving a throat, wherein an aqueous scrubbing liquid comprisingpreferably 20 wt. % to 55 wt. % urea is sprayed into the gas stream inthe direction of the throat,

D. spraying an aqueous solution into the gas stream, preferably inco-current flow or cross-flow, preferably with a solution comprising 0wt. % to 5 wt. % urea, and optionally comprising an acid, for examplespraying with water,

E. optionally passing the gas stream through a mist eliminator,

F. passing the gas stream through a second ejector Venturi scrubberhaving a throat, wherein an aqueous scrubbing liquid comprisingpreferably 0 wt. % to 5 wt. % urea is sprayed into the gas stream in thedirection of the throat,

G. optionally spraying an aqueous solution into the gas stream,optionally comprising 0 wt. % to 5 wt. % urea, and/or comprising acid orbase reagent, optionally in co-current flow, cross-flow orcounter-current flow, and

H. optionally passing the gas stream through a mist eliminator.

Usually, this preferred method comprises passing the gas stream throughat least one mist eliminator after at least one Venturi stage.Preferably, this method is carried out in a system on top of a prillingtower, the system comprising the Venturi stages. Preferably in sprayingstep D, second Venturi stage step F and/or step G, the solution and/orliquid comprises less than 2 wt. % water, and optionally consists ofwater, and at least 5 g water/Nm³ is evaporated in these steps.Optionally, further steps are included, for example a mist eliminationstep upstream of step D. Possibly one or more or all of said steps arecarried out consecutively. The invention also pertains to a gas streamtreatment method comprising these steps.

The invention generally also pertains to a gas stream treatment systemcomprising at least one Venturi ejector. The Venturi ejector maycomprise a Venturi scrubber and upstream thereof a nozzle directed tothe throat of the Venturi scrubber, further comprising a pump in fluidconnection with said nozzle for pressurizing at least liquid supplied tosaid nozzle. Venturi ejectors with round throats as well as withrectangular throats can for example be used.

The system preferably comprises two Venturi ejector stages in series, asdescribed.

A preferred gas stream treatment system, preferably for a methodaccording to the invention, comprises two Venturi stages in series,wherein each of said two Venturi stages comprises a horizontally placedVenturi ejector comprising a converging part, a throat, and a divergingpart, and a nozzle for spraying into said throat, wherein said Venturistages are placed above each other. Spraying includes for exampleinjecting a liquid jet which breaks up into a spray, such that a sprayis provided into said throat. Preferably, the horizontally placedVenturi ejectors extend at least in part into a scrubbing column.Preferably the Venturi ejectors extend at least in part, or entirely,under or above one of the mist eliminators and/or above at least one ofthe basins (reservoirs) for collecting liquids. Preferably the twoVenturi stages are connected by a scrubbing column. Preferably thesystem comprises two adjacent scrubbing columns integrated in a singlecasing.

A preferred gas stream treatment system comprises two Venturi stages inseries, with a spray section in between, wherein the spray section ismore preferably for spraying a fine mist so as to provide forevaporation of sprayed liquid downstream a Venturi stage and upstream ofa Venturi stage. Optionally, the system comprises a compressor in fluidcommunication with a dual fluid nozzle for providing compressed air.Preferably, the gas stream treatment system comprises a mist eliminatorbetween the two Venturi stages, and preferably a second mist eliminatordownstream of the second Venturi stage. Optionally, these features arecombined with the mentioned system having horizontally placed Venturiejectors.

Preferably each Venturi stage comprises a scrubbing liquid recirculationloop, in particular comprising a pump for pressurizing and recirculatingscrubbing liquid to said nozzle. Preferably, the Venturi stages haveseparate recirculation loops. The separate recirculation loops allow fordifferent chemical compositions of the scrubbing liquids of each stage.A recirculation loop may comprise a fluid communication line from acollection basin or sump where scrubbing liquid is collected to one ormore spraying nozzles.

In a further preferred embodiment, which can be combined, the gas streamtreatment system comprises a horizontally placed ejector Venturiscrubber comprising an open-ended converging tube or duct part, athroat, and an open ended diverging part and a spray nozzle positionedinside said converging part for spraying into said throat, and whereinthe open end of said converging part is a gas inlet for the gas streamto be treated.

Preferably, the system comprises a sprayer between a first and adownstream second Venturi stage, for spraying an aqueous solution havinga higher water concentration (i.e. lower concentration dissolved andparticulate material) than the scrubbing liquid of the first Venturistage.

Optionally, the gas stream treatment system has a fluid connection witha urea finishing section, in particular for off-gas of a ureagranulation section of urea prilling tower.

Optionally, the gas stream treatment system is located at the top of aurea prilling tower, such as on top of the tower. The invention alsopertains to a urea prilling tower having a gas stream treatment systemcomprising a Venturi ejector at the top of the prilling tower, morepreferably comprising two Venturi ejectors in series, and even morepreferably a gas stream treatment system as described.

Optionally, the system does not comprise a fan or blower. Preferably,the system does not comprise a fan or blower for producing a pressuredrop of the gas stream.

Advantages of the method and system of the invention include a lowpressure drop, good efficiency for removal of submicron particles, andcompact design. Pressurizing a liquid such as the recirculatingscrubbing liquid can be efficiently done using compact equipment, e.g. apump. Atomizing the liquid with compressed air can also be efficientlydone using compact equipment, e.g., a compressor. Further advantageousperformance is achieved through the preferred inclusion of multiplestages employing progressively less concentrated aqueous solutions (e.g.lower urea concentration) to promote particulate growth by condensationon the surface of submicron particulate.

The invention also pertains to a method of modifying an existing plant,such as a urea finishing section and/or prilling tower, in particular aurea prilling tower, the method comprising adding a gas stream treatmentsystem as described. The invention also pertains to a method ofmodifying an existing urea finishing section and/or prilling tower, inparticular an existing urea prilling tower, the method comprising addinga gas stream treatment system comprising a Venturi ejector, preferablyon top of the prilling tower. Preferably the gas stream treatment systemcomprises two Venturi stages in series. Preferably the system is asystem as described and/or for the described methods. The invention alsopertains to a method of modifying an existing urea finishing sectionhaving a Venturi ejector, comprising adding a Venturi ejector in serieswith said Venturi ejector.

Referring now to the drawings in general, the illustrations are for thepurpose of describing a preferred embodiment of the invention andillustrating preferred features of the systems and methods, and are notintended to limit the invention.

FIG. 1 shows a non-limiting embodiment of the invention. The vessel 1contains all of the components. Provided is an inlet zone for the gasstream (e.g. at 90% to 110% ambient pressure) with spray nozzles 2.Spray nozzles 2 spray in cross-flow and downward. Sprays 2 may spray asolution obtained from reservoir 4. Spray nozzles 2 may provide forquenching of the gas stream. Sprays 2 may provide for condensation ofwater on submicron particles and/or for scrubbing. Sprayed liquid withcaptured and dissolved particles is collected in reservoir 4 under spray2. Further provided is Venturi ejector 3, e.g. a Venturi eductor 3.Venturi ejector 3 comprises a nozzle. The nozzle sprays scrubbing liquidinto the throat. The gas inlet of ejector 3 is parallel to the openingof throat 3 and is vertical. Ejector 3 has a horizontal orientation. Thegas inlet of the gas inlet zone 2, the gas inlet of the ejector 3, andthe gas outlet of Venturi ejector 3 are all on a line, contributing tolow pressure drop. The nozzle is positioned inside the converging tubepart, downstream of and spaced from the gas inlet of the Venturi ejector3. At the bottom of the gas inlet zone 2, a concentrated water reservoir4 is provided, which is also provided in the second column. From thestream obtained from the outlet of ejector 3, droplets are removed, e.g.using a mist eliminator 5, and the droplet including captured urea iscollected in reservoir 4. The liquid in reservoir 4 is recirculatedthrough a loop to Venturi ejector 3 (with pressurizing) and optionallyto spray 2. A purge stream is withdrawn from reservoir 4 and disposedof. The mist eliminator comprises a mesh and upstream thereof aplurality of spray nozzles.

These nozzles spray optionally in co-current direction and optionally asfine mist, for instance with droplet size of less than 300 μm or lessthan 200 μm. They typically spray dilute water, e.g. from reservoir 7.The spray nozzles upstream of the mist eliminator for instance spray anaqueous solution comprising less than 5 wt. % urea, such essentiallywater. The compartment between Venturi ejector 3, or said spray nozzles,and second Venturi ejector 6 preferably provides sufficient residencetime to allow for evaporation of e.g. at least 50 wt. % of the sprayedwater and for condensation on submicron particles.

Further provided is a second Venturi eductor 6 for spraying with dilutescrubbing liquid. The arrangement of the nozzle is the same as foreductor 3, hence parallel to the gas flow. Also provided is a dilutewater reservoir 7. Reservoir 7 can be provided with a recirculation loopcomprising a pump to the spray nozzle of eductor 6. Also provided is amist eliminator 8 for eliminating droplets e.g. as formed in the Venturieductor. Mist eliminator 8 may comprise a mesh, chevron, or combinationsof each. Mist eliminator 8 is shown with horizontal arrangement. Anyarrangement, including vertical, of a mist eliminator is also possible.

Further provided is a scrubber upstream of said mesh for spraying liquidcounter-current into said gas flow. Cross-flow and co-current flowspraying are also possible. The sprays can also be directed to the misteliminator cleaning the mist eliminator 8. Reservoir 7 collects dropletsfrom eductor 6 and from said scrubber. A purge stream from reservoir 7can be disposed of e.g. by supplying to reservoir 4, in view of theevaporation of liquid from reservoir 4 in for instance spray 2. Acid orbasic reagent is optionally used for scrubbing in the scrubberdownstream of Venturi eductor 6. Also provided is an exit duct 9 forventing to the environment.

In operation, the particle-laden gases enter the inlet zone 2 where hotgases are initially cooled by evaporation of particle-concentrated water(e.g. an aqueous urea solution) from spray nozzles 2. The gases enterthe first Venturi eductor 3 where the liquid motive force from sprayingscrubbing liquid through the nozzle propels the gases forward withoutrequiring a pressure drop. In the first Venturi eductor 3, the gases arescrubbed with the scrubbing liquid. The gas stream typically is orbecomes saturated with water, based on the partial water vapor pressureof the scrubbing liquid. Coarse particulate is collected and dissolvedinto the water and further concentrated in the concentrated waterreservoir 4. A gas stream including droplets leave the first Venturieductor 3 and pass through a mist eliminator 5 including thespraying/scrubbing. The gas stream passes through the mist eliminator 5then enters the second Venturi eductor 6. Liquid from spray nozzles andmist eliminator 5 and from the outlet of Venturi ejector 3 is collectedin reservoir 4 at the bottom of the scrubbing column. The water inreservoir 4 comprises e.g. at least 40 wt. % dissolved material and e.g.at least 80 wt. % of the total removed particulate matter (includingscrubbed in spray 2). In the second Venturi eductor 6, optionally atleast some water evaporates, optionally saturating the gases with morewater. In each Venturi eductor, gas and liquid are mixed and typicallyparticles are entrained in liquid droplets, and the droplets are removedfrom the gas stream downstream of the Venturi eductor. Gases continuethrough the second mist eliminator 8, including the spraying, and thenexit the scrubber at exit duct 9. Liquid obtained from mist eliminator 8is collected in reservoir 7 and may be recirculated to Venturi eductor6. The liquid in reservoir 7 comprises e.g. less than 5 wt. % urea ande.g. less than 20 wt. % of total urea removed from the gas stream.

The Venturi eductors (3, 6) are preferably mounted in a horizontalposition thereby allowing a compact design minimizing height, andpreferably above each other to reduce footprint.

The material of construction of the scrubber casing and the venturieductors is optionally a light-weight material such as FRP (FiberReinforced Plastic).

The pressure drop over the scrubber is typically less than 250 Pa, butcan be zero, depending on the inlet and outlet ducting pressure drop. Inmany instances, there will be a net pressure gain, for example of up to500 Pa or even more, with the outlet pressure being higher than in theinlet pressure.

FIG. 2 shows an embodiment wherein the system comprises a plurality ofparallel Venturi ejectors (3) and a second Venturi stage with aplurality of parallel Venturi ejectors (6). Herein the scrubbing columnand the venturi are integrated in a single casing. Venturi ejector (3)extends horizontally under mist eliminator 5 and/or reservoir 7. Thesecond Venturi ejector (6) stage extends horizontally above misteliminator (5) and/or reservoir (7). A wall is provided between the twoparts, at both sides in contact with process streams, separating the twohalves, and only allowing for gas flow from one side to the other sidethrough the Venturi ejectors (3,6). The diverging part of the Venturiejectors (3, 6) is provided at the upper side with an elbow forgas/liquid separation having a bottom outlet between the diverging partend and a vertical separation.

FIG. 3 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a prilling tower.

FIG. 4 shows a typical particle size distribution and cumulative massfor urea dust in off-gas from a urea granulator.

The invention will now be further illustrated by one or more examples,which do not limit the invention.

EXAMPLE 1

As an example of a preferred embodiment, a prilling tower is proposedthat has a urea-laden airflow that needs to be scrubbed. The temperatureof the air leaving the prilling tower is 80° C., the molar fraction ofwater vapor is 2%, and the concentration of urea dust is 25 μg/g of gasflow. A Venturi eductor (ejector) spray is provided which will cool theair by evaporation until the airflow is saturated and water no longerevaporates. Using thermodynamic calculations in combination with steamtables, it is determined that this will occur at a final gas temperatureof 33° C. with a water vapor molar fraction of 2.5% when using purewater. For this proposed project the amount of water evaporated iscalculated to be 0.03 kg/Nm³ of gas flow. However, in practice, theVenturi eductor spray will be recycled until the urea concentrationapproaches 45% wt. At this urea concentration, the vapor pressure ofwater is proportionally less. Using Raoult's law, the above calculationsare repeated to find that the new saturated gas temperature is 37° C.with a water vapor molar fraction of 2.2%. Even though the saturatedtemperature is 4° C. higher, the molar fraction of water in the gasstate is more than 10% less. Only 0.02 kg of water per Nm³ of gas flowis predicted to evaporate, for the proposed project. Downstream of theconcentrated Venturi eductor spray, when the gases are exposed to dilutewater (e.g. aqueous urea solution having a lower urea concentration,such as essentially no urea), the saturated conditions will match thefirst case, enabling an additional 0.01 kg/Nm³ of evaporation. By usinga second dilute water spray that is preferably at least 0.2 seconds, orpreferably 0.3 seconds, upstream of the dilute Venturi eductor (secondVenturi stage), submicron particulate growth is promoted. This improvesparticle capture in for example a Venturi eductor. The dilute waterspray is for example implemented as spray in a mist eliminatordownstream of first Venturi stage and upstream of a second Venturistage, allowing for condensation on submicron particles. A quench sprayupstream of a first Venturi stage may promote initial condensation onsubmicron particles.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive; theinvention is not limited to the disclosed embodiments.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims and specification, the word “comprising”does not exclude other elements or steps, and the indefinite article “a”or “an” does not exclude a plurality. Terms such as “usually”,“generally”, “typically”, “in particular”, “can”, “may” and “suitably”indicate non-essential features which may be omitted in some embodimentsand can be combined with preferred features. The mere fact that certainfeatures of the invention are recited in mutually different dependentclaims does not indicate that a combination of these features cannot beused to advantage. Features of the methods can be combined with featuresof the system and features of the embodiments can be combined withfeatures illustrated in the drawings. The preferred methods can forexample be carried out in preferred systems and apparatuses.

1. A method for removing submicron particles from a gas streamcomprising: a) providing a gas stream, b) spraying a first aqueoussolution into the gas stream, c) passing the gas stream from b) througha first ejector Venturi scrubber having a throat, and spraying a secondaqueous scrubbing liquid into the gas stream in the direction of thethroat, d) spraying a third aqueous solution into the gas stream thathas passed through said first ejector Venturi scrubber, and e) passingthe gas stream from d) through a second ejector Venturi scrubber havinga throat, and spraying a fourth aqueous scrubbing liquid into the gasstream in the direction of the throat.
 2. The method of claim 1, whereinthe gas stream comprises off-gas from a urea prilling tower, and thefirst aqueous solution comprises 20 wt. % to 55 wt. % urea, the secondaqueous scrubbing liquid comprises 20 wt. % to 55 wt. % urea, the thirdaqueous solution comprises 0 wt. % to 5 wt. % urea, further comprisingpassing the gas stream from d) through a mist eliminator, the fourthaqueous scrubbing liquid comprises 0 wt. % to 5 wt. % urea; and f)spraying a fifth aqueous solution comprising 0 wt. % to 5 wt. % ureainto the gas stream from e), and further comprising passing the gasstream from f) through a mist eliminator.
 3. The method of claim 2wherein, in step b) the gas stream is cooled by at least 10° C.
 4. Amethod for removing submicron particles from a gas stream, the methodcomprising: contacting a gas stream containing submicron particles in aVenturi ejector with an injected high velocity scrubbing liquid toprovide a pumping action, wherein the scrubbing liquid has an initialvelocity of at least 25 m/s and wherein the ratio of scrubbing liquidand gas flow is between 0.0005 and 0.0015 (m³/h)/(m³/h).
 5. The methodof claim 4, wherein the gas stream is the off-gas from a urea granulatoror a urea prilling tower.
 6. The method of claim 4, wherein the methodcomprises using a plurality of Venturi ejectors in series.
 7. The methodof claim 6, wherein the residence time between the first and adownstream second Venturi ejector is at least 0.4 s.
 8. The method ofclaim 4 wherein said residence time is more than 0.8 s.
 9. The method ofclaim 4, wherein the gas stream contains a concentration of submicronparticles greater than 20 mg/Nm³.
 10. The method of claim 4, wherein theVenturi ejector comprises a nozzle positioned for spraying scrubbingliquid in a direction parallel with the gas flow through the gas inletof the Venturi ejector.
 11. The method of claim 4, wherein the methodcomprises acid scrubbing and dust scrubbing.
 12. The method of claim 11,wherein the acid scrubbing and dust scrubbing are both carried out atthe top of a natural draft urea prilling tower
 13. The method claim 4,further comprising the step of urea prilling in a natural draft ureatower and wherein said Venturi ejector is positioned at the top of saidprilling tower.
 14. The method claim 4, wherein the static pressure atthe exit of at least one Venturi ejector is larger than at the entranceof the Venturi ejector.